pupil vs college boy

pupil and college boy both are nouns.

Word	Noun	Adjective	Verb	Adverb
pupil	Yes	No	No	No
college boy	Yes	No	No	No

As nouns, college boy is a hyponym of pupil; that is, college boy is a word with a more specific, narrower meaning than pupil:

pupil (noun)	college boy (noun)
the contractile aperture in the center of the iris of the eye; resembles a large black dot	a student (or former student) at a college or university
a learner who is enrolled in an educational institution
a young person attending school (up through senior high school)